U.S. Pat. No. 3,885,635 entitled "Two Speed Mechanical Quill Feed and Spindle Drive Mechanism for a Machine Tool", herein incorporated by reference and sometimes referred to as the "Mechanical Quill Patent", describes a mechanical quill drive system having many similarities to the present invention. In that patent, a quill is advanced by an external screw cooperating with a ball nut having a two speed drive. The two speed drives includes a motor driving the nut directly through gearing during rapid infeed of the quill, but which motor is itself driven through a clutch and reduction gearing by a second motor to produce the slow-speed infeed of the quill and which second motor also rotates the spindle by a driving connection with the splined drive shaft at a reduction during infeed. U.S. Pat. No. 4,000,661 is a division of U.S. Pat. No. 3,885,635.
In some applications for such quill systems, it is desirable to allow the spindle motor to run continuously, without advancing or retracting the spindle. Such spindle operation minimizes the sophistication and cost of the motor controls, but is not possible in the apparatus described in the Mechanical Quill Patent.
In order applications it is desirable to mount the quill unit with the spindle extending vertically. In such cases the mass of the quill and spindle have a tendency to cause undesirable or downward movement ("drift") of the spindle.
In some applications it is desirable to interrupt the feed stroke for chip breaking or dwelling at the completion of the stroke to assure a good finish of blind holes. The Mechanical Quill Patent cannot conveniently perform this task without substantial modification of the motor controls. Without a dwell capability, an undesirable burr is frequently created in machining.
Other feed mechanisms for quill shafts of a type used in machine tools have been in use for many, many years. While quite satisfactory hydraulic design approaches have existed in the past, current trends are for various reasons dictating a "mechanical" approach to the quill unit drive. Such mechanical drives developed to date have various drawbacks.
One such approach involves a screw actuator disposed parallel to the quill shaft which is rotated at two speeds to provide the two-speed advance of the quill shaft. An example of this design is found in U.S. Pat. No. 3,561,544. Another somewhat similar approach common also in single speed drill press type applications has involved a rack or cam drive of the quill shaft; such as shown in U.S. Pat. Nos. 2,545,008 and 2,902,891, as well as in German Pat. No. 846,807.
These approaches have the common difficulty of excessive runout of the spindle shaft since the feed forces are not aligned with the axis of the motion of the quill shaft. While some attempts to overcome this problem have been made by a power screw arrangement concentric with the quill shaft as exemplified by U.S. Pat. Nos. 2,796,767; 2,975,440; 3,430,524; and 3,139,637, these designs require bearings on a spindle shaft forward of the screw shaft and hence are quite cumbersome, and runout problems may also exist due to the long unsupported screw portion overhung at one end of the quill shaft of the mechanism, and also frictional loads may be quite high.
A concentric ball screw advancing mechanism has been described in U.S. Pat. No. 3,640,147 which would substantially alleviate the friction loads, but this design provides only a single speed feed motion and the screw portion is, similar to the above examples, overhung at one end of the quill shaft necessitating a critical bearing adjustment arrangement to control runout.
This particular means for providing a two-speed feed drive previously proposed have also not been without significant drawbacks.
A common approach seen in some of the aforementioned patents as well as U.S. Pat. No. 3,283,664 has been to provide a pair of motors driving through either a planetary or screw differential gearing. Such gearing is expensive and the controls tend to be quite complex.
Another approach is to provide a variable speed commutated motor driven at variable speed, but such motors as compared to constant speed induction motors require more maintenance (due to the brushes) and require expensive and complex electrical or electronic control systems.
Another approach is shown in U.S. Pat. No. 4,201,271 but requires costly controls.
Accordingly, it is an object of the present invention to provide a two-speed quill mechanical unit drive with a selective brake which allows mounting the quill vertically, continuous operation of the spindle motor and interrupted feed (dwell) without expensive controls.
It is another advantage of the present invention to provide a reliable two-speed drive with a minimum of gearing and controls which is capable of accurately controlling the point at which the shift to the lower speed infeed is carried out. Other objects and advantages will be apparent to those skilled in this art in view of the following description of the invention of the drawings.